RegExMatchArray.groups does not allow an undefined value

[PartMan7]

🔎 Search Terms

"regexp", "groups", "capture groups"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about key types and object types.
• I was unable to test this on prior versions to 4.0.5 because RegExpMatchArray.groups was added then.

⏯ Playground Link

https://www.typescriptlang.org/play/?target=99&ts=5.9.0-dev.20250324#code/MYewdgzgLgBAtgQysAFjAvDA5ArA6RZFACgHpiB+AHgBsBTKKOgJwD4BtBAWgC8BdAJQAfSlTABXOACMWrADoATAaQEBuAFABLAGYxihVBTwBzZiHEAHCAJgBvdTEcxQkWKfNWM8JKhNnLEDAIgbYw9IwsAFww0MyaYMaqMBLSUTDiYAp02vF0CjAAvjBCdmEMTMzRGVk5YHlJKTKVMVBxCYVJpKTOCGBYsAqazHTAUDQAnjBQ4xYjwVAOTi4QIPR4NCDGxO4BaouOOnpYjSxYMPHeRH4e1s7gK2sbWwYo1wF4J8zrdAlQKGowLrOZjBFB0CDqApAA

💻 Code

const match = 'a'.match(/(?<letter>[a-z])|(?<number>\d)/);
if (match?.groups) {
    const groups = match.groups as { letter: string; number: undefined } | { letter: undefined; number: string }; // can't directly typecast
    console.log(groups);
    if ('number' in match.groups) console.log(match.groups.number.length); // crashes without warning
}

🙁 Actual behavior

• TypeScript has RegExpMatchArray.groups typed as { [key: string]: string } instead of potentially { [key: string]: string | undefined }, and does not allow typecasting it to an object with undefined as a value.

---

• TypeScript does not allow casting Match.groups as an object whose keys may be undefined (eg: { group1: string; group2: undefined })
• TypeScript assumes the in operator to narrow the type due to undefined not being a valid value, despite unmatched named groups in alternation in RegEx returning undefined.

🙂 Expected behavior

• undefined should be allowed in the value type (for typecasting correctly or for correctly not narrowing with in).

Additional information about the issue

Checked #32098 but that looks at inferring types, not allowing casting.

[MartinJohns]

Casting is allowed when you use a compatible type: { [group: string]: string | undefined }
You can't use your type with the known properties due to #32098 not being supported. Alternatively you can first cast to unknown, then to your custom type.

[PartMan7]

I get why it happens, but it's incorrect from both standpoints (the value can be undefined even if the key is in the object, and I shouldn't need to cast to unknown when my type is a subset of the intended type). I understand that match.groups as { [group: string]: string | undefined } as { letter: string; number: undefined } | { letter: undefined; number: string } works, but that's just a workaround, not a solution (at least to me).

My issue here is TypeScript reporting the captured group type invalidly (ie; asserting that value will always be string even when it can also be undefined).

[MartinJohns]

I shouldn't need to cast to unknown when my type is a subset of the intended type

But it isn't a subset, they're incompatible types. { letter: string; number: undefined } | { letter: undefined; number: string } is not a subset of { [key: string]: string }.

[jcalz]

Duplicate #59147, #51292, #49228, #17053, etc

[PartMan7]

I shouldn't need to cast to unknown when my type is a subset of the intended type

But it isn't a subset, they're incompatible types. { letter: string; number: undefined } | { letter: undefined; number: string } is not a subset of { [key: string]: string }.

You're speaking of the types in play here; I'm trying to say that TypeScript's provided type here is incorrect in the first place (and that the correct type would let this code work). Regardless, it seems I'd missed the closed issues that @jcalz mentioned and so this probably won't go anywhere. Thanks!

[MartinJohns]

You're speaking of the types in play here; I'm trying to say that TypeScript's provided type here is incorrect in the first place (and that the correct type would let this code work).

.. which would first require #32098, as mentioned.

[PartMan7]

(32098 deals with inferring the types from the given RegEx, which is something else entirely)